1. Field of the Invention
The present invention relates to a capacitor module and a semiconductor device using the capacitor module, and more particularly to a capacitor module used to construct an inverter device and to a semiconductor device using the capacitor module.
2. Description of the Related Art
Inverters are widely used in various kinds of consumer-oriented or industrial electronic appliances. For example, in an electric vehicle propelled by an a.c. motor or a hybrid car propelled by an internal combustion engine and an a.c. motor, an inverter 101 (hereinafter referred to as related art 1) is interposed between the motor and a d.c. power supply, as shown in FIG. 20. As shown in a plan view of FIG. 18 and a cross-sectional view of FIG. 19, the inverter 101 is constituted by a semiconductor device 102 and a smoothing capacitor 110 placed outside the semiconductor device 102. The smoothing capacitor 110 is required to reduce ripple voltage changes in the d.c. power supply. The semiconductor device 102 coverts d.c. current into a.c. current by switching devices 120 and diodes 121 mounted on an insulating board 125 or, conversely, converts a.c. current into d.c. current. If a three-phase a.c. motor is used, the semiconductor device 102 has three phases: a U phase 140, a V phase 141, and a W phase 142. The insulating board 125 is mounted on a heat radiation plate 160 which is fixed on a case 150 formed of a synthetic resin. A plurality of conductors for internal wiring are embedded in the case 150 by insert molding. The conductors have exposed their portions in the surface of the case 150, which form a P terminal 130 and an N terminal 131 on the d.c. side and a U terminal 132, a V terminal 133, and a W terminal 134 on the a.c. side. Also, the conductors are connected to the switching devices 120 and diodes 121 by a wiring pattern and aluminum wires (not shown) formed on the surface of the insulating board 125, thereby forming the circuit shown FIG. 20. A d.c. power supply is connected to the P terminal 130 and the N terminal 131. A three-phase a.c. motor is connected to the U terminal 132, V terminal 133, and the W terminal 134 on the a.c. side.
As mentioned above, the smoothing capacitor 110 is provided outside the semiconductor device 102 when the semiconductor device constitutes the inverter in the related art 1. For this reason, the wiring lines between the smoothing capacitor 110 and the switching devices 120 in the semiconductor device 102 are long and the inductance thereof is large. A high surge voltage can be caused under such a condition. Therefore there is a need to increase a withstand pressure of the semiconductor elements and an increase in manufacturing cost is inevitable. Since the inductance is increased, it is necessary to increase capacitance of the smoothing capacitor 110 in order to reduce ripples in the voltage of the d.c. power supply. Therefore, the smoothing capacitor 110 must be increased in size, resulting in an increase in overall size of the inverter 101.
Ordinarily, an electrolytic capacitor in a cylindrical form or the like is used as a capacitor having large capacitance. If such a capacitor is used, it is difficult to efficiently use the space. This is a hindrance to reducing the size of the inverter 101.
Japanese Patent Application Laid-open No. 10-304680 discloses use of a ceramic capacitor as a smoothing capacitor to reduce the size of a semiconductor device, and a structure in which the ceramic capacitor is placed in the vicinity of switching devices inside the semiconductor device (related art 2). FIGS. 21 to 23 show the configuration of a conventional power converter device described in the specification disclosed in this publication.
In an embodiment of the power converter device disclosed in Japanese Patent Application Laid-open No. 10-304680, a ceramic capacitor C is used as a smoothing capacitor and mounted on a switching device board 226 on which insulated gate bipolar transistors (IGBTs), etc., are mounted. The ceramic capacitor C is cooled with a cooling member 218, with which the IGBTs, etc., are also cooled. More specifically, as shown in FIG. 22, the ceramic capacitor C having the shape of a substantially rectangular block is placed horizontally position between power supply wiring conductors on the plus and minus sides (hereinafter referred to as P-polarity conductor 236P and N-polarity conductor 236N). Alternatively, the ceramic capacitor C is placed vertically, as shown in FIG. 23. Three ceramic capacitors connected in parallel with each other may be provided in one-to-one relationship with the three phases to realize a smoothing capacitor.
One of the advantages of use of a ceramic capacitor as a smoothing capacitor is that a ceramic capacitor has an internal resistance smaller than that of electrolytic capacitors and enables limitation of the capacitance to a necessary value for smoothing, while in the related art the capacitance is set to a comparatively large value for absorption of a ripple voltage. More specifically, the necessary capacitance of the smoothing capacitor can be limited to several hundred microfarads, while the necessary capacitance in the related art is several ten millifarads. Consequently, the smoothing capacitor can be reduced in size.
The above-described structure has a problem relating to a method of connection between the ceramic capacitor C and each of the P-polarity conductor 236P and the N-polarity conductor 236N. A case will be discussed where three ceramic capacitors connected in parallel constitute a smoothing capacitor in the manner disclosed in the above-mentioned publication in the described example of the inverter device mounted in an electric vehicle.
In the specification disclosed in the above-mentioned publication, it is stated that the capacitance necessary for smoothing can be limited to several hundred microfarads if a ceramic capacitor is used as a smoothing capacitor. However, the external size of one ceramic capacitor in a case where three ceramic capacitors are connected in parallel with each other as described in the disclosed specification to realize such capacitance is thought to be at least several ten millimeters square.
The method of connecting the ceramic capacitor C and each of the P-polarity conductor 236P and the N-polarity conductor 236N is not described in detail in the above-mentioned publication, but the ceramic capacitor C and each of the P-polarity conductor 236P and the N-polarity conductor 236N in the state as understood from FIGS. 22 and 23 are connected to each other with their surfaces facing each other. From the viewpoint of mounting on a electric vehicle, it is thought that it is necessary for the connected surfaces to be maintained in the connected state with reliability even when they are caused to vibrate, and it is also necessary for the connected surface to be not only in contact with each other but also in a state of being firmly fixed to each other. Also, while it is necessary to apply a substantially high pressure to the contact surfaces in order to ensure reliable connection by contact, no devise is made to apply a contact pressure to the contact surfaces in the art as understood from the disclosure in the above-mentioned publication, and it can easily be conjectured that the art was proposed with mere fixation of the connected surfaces imagined.
Further, to make the best possible use of the capacitance of a ceramic capacitor, it is necessary to maximize the uniformity of the current density in the ceramic capacitor. For this effect, it is necessary that each of the P-polarity conductor 236P and the N-polarity conductor 236N be connected to substantially the entire surface of an external electrode of the ceramic capacitor, or that the connection be distributed uniformly on substantially the entire area of the external electrode of the ceramic capacitor.
Ordinarily, a metal such as copper having a high electrical conductivity and low-priced is used as the material of the P-polarity conductor 236P and the N-polarity conductor 236N to which the ceramic capacitor is connected.
For the above-described reasons, it is required for implementation of the related art disclosed in the above-mentioned publication that materials differing in the liner expansion coefficient, i.e., a ceramic and a metal, be connected in such a state that the area of contact therebetween is several ten millimeters square. In implementation of the related art under this requirement, occurrence of considerable thermal stress in portions of the two members jointed to each other cannot be avoided. For example, in the case of the inverter device mounted in an electric vehicle, which is described as an example in the specification disclosed in the above-mentioned publication, the inverter device has an operating temperature range from −40 degrees to 125 degrees and the components are subjected to repeated thermal action due to variation in temperature in this range. In such a situation, it is inevitable that the joint or the ceramic capacitor itself is seriously damaged by thermal stress caused by the thermal action.
Further, in a case where a multilayer ceramic capacitor, e.g., one using a barium titanate ceramic as a dielectric is used at a high voltage or in a high frequency region in particular, electrostriction can occur easily due to a piezoelectric phenomenon of the dielectric provided in the capacitor main body. The amount of electrostriction is particularly large if the capacity of the multilayer ceramic capacitor is large. If, in a situation where such electrostriction is caused, the connection members are joined to the external electrodes in a state of having the joint surfaces faced to each other in the manner described with respect to the related art disclosed in the above-mentioned publication, displacement of the capacitor body due to electrostriction is restricted comparatively strongly by the connection members to reduce the escape of the stress due to electrostriction by a comparatively large amount. There is a possibility of damage to the ceramic capacitor resulting from such a condition.
However, it is thought that the structure disclosed in Japanese Patent Application Laid-open No. 10-304680 was designed with no consideration of such stress due to heat or electrostriction.
Japanese Patent Application Laid-open Nos. 2000-223355 and 2000-235931 disclose structures (referred to as related arts 3 and 4, hereinafter) which were designed to avoid problems of thermal stress and electrostriction such as those described above, and in which a terminal member made of a metal plate is provided as the external electrode of the ceramic capacitor to reduce, by deformation of the terminal member including bending, stress acting on the joint and the ceramic capacitor main body. In the art disclosed in Japanese Patent Application Laid-open No. 10-304680 however, no application of a ceramic capacitor having such a terminal member is supposed. No guide to a method of application of such a ceramic capacitor can be obtained from the related art. Also, ceramic capacitors disclosed in Japanese Patent Application Laid-open Nos. 2000-223355 and 2000-235931 are assumed to be connected to a planar member such as a printed board without supposition of interposition between conductors opposed to each other as shown in FIGS. 22 and 23 in Japanese Patent Application Laid-open No. 10-304680. No guide to a method of such application can be obtained from these related arts.
Further, if P-polarity and N-polarity conductors are placed along a plane in an application of the ceramic capacitor disclosed in Japanese Patent Application Laid-open No. 2000-223355 or 2000-235931, the ceramic capacitor is in a horizontal position. Note that the term “horizontal position” refers to a state in which the ceramic capacitor is positioned so that one of the surfaces of the ceramic capacitor having the largest area (referred as a major surface, hereinafter) is horizontally arranged. Alternatively, the ceramic capacitor may be in a vertical position. The major surface of the ceramic capacitor in this position is perpendicular to the surface on which the ceramic capacitor is mounted. If the size of the ceramic capacitor is several ten millimeters square, and if the ceramic capacitor is in the horizontal position, the size of the semiconductor device is considerably large. To avoid this, the ceramic capacitor is vertically positioned, or placed above the power converter circuit. However, it is difficult for each of the ceramic capacitors disclosed in Japanese Patent Application Laid-open No. 2000-223355 or 2000-235931 to be connected to the P-polarity and N-polarity conductors while being maintained in a position other than the horizontal position. Thus, the degree of freedom of positioning the capacitor is low.
Even if portions of the P-polarity and N-polarity conductors are raised upright as shown in FIG. 23 in the art disclosed in Japanese Patent Application Laid-open No. 10-304680, it is necessary to devise some means for enabling joining of the ceramic capacitor to the vertical surfaces, e.g., means for supporting the ceramic capacitor before the ceramic capacitor is connected and fixed, or a method of changing the orientation of the semiconductor device to horizontally maintain the portion to which the ceramic capacitor is connected. In such a case, troublesome operations are required and an increase in manufacturing cost of the semiconductor device are caused.
The ceramic capacitor and the terminal members are connected by soldering. If the same solder as that for the connection between the ceramic capacitor and the terminal members is used to connect the ceramic capacitor and the P-polarity and N-polarity conductors, there is a risk of the solder for the connection between the ceramic capacitor and the terminal members being molten to allow shifting of the joint positions or disconnection of the ceramic capacitor and the terminal members.
In the related art 1, as described above, the wiring lines between the smoothing capacitor and the switching devices are long, the inductance thereof is large, and there is a need to increase the capacitance of the smoothing capacitor, so that the size of the smoothing capacitor is increased. In the related art 2, a ceramic capacitor is therefore used to achieve a reduction in size but there is a possibility of the ceramic capacitor being broken when stressed by thermal stress or electrostriction since the ceramic capacitor and each of the P-polarity conductor and the N-polarity conductor are connected with their surfaces facing each other. Each of the related arts 3 and 4 is a certain measure of success in solving the stress problem. In each of these arts, however, the degree of freedom with which the capacitor is positioned when mounted in a semiconductor device or the like is low and an increase in size of the semiconductor device cannot be avoided.